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![Uros Bizjak](/assets/img/avatar_default.png)
Use atomic_try_cmpxchg() instead of atomic_cmpxchg(*ptr, old, new) == old in rcuref_put_slowpath(). On x86 the CMPXCHG instruction returns success in the ZF flag, so this change saves a compare after CMPXCHG. Additionaly, the compiler reorders some code blocks to follow likely/unlikely annotations in the atomic_try_cmpxchg() macro, improving the code from: 9a: f0 0f b1 0b lock cmpxchg %ecx,(%rbx) 9e: 83 f8 ff cmp $0xffffffff,%eax a1: 74 04 je a7 <rcuref_put_slowpath+0x27> a3: 31 c0 xor %eax,%eax to: 9a: f0 0f b1 0b lock cmpxchg %ecx,(%rbx) 9e: 75 4c jne ec <rcuref_put_slowpath+0x6c> a0: b0 01 mov $0x1,%al No functional change intended. Signed-off-by: Uros Bizjak <ubizjak@gmail.com> Signed-off-by: Ingo Molnar <mingo@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Paul E. McKenney <paulmck@kernel.org> Link: https://lore.kernel.org/r/20230509150255.3691-1-ubizjak@gmail.com
282 lines
9.5 KiB
C
282 lines
9.5 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* rcuref - A scalable reference count implementation for RCU managed objects
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*
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* rcuref is provided to replace open coded reference count implementations
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* based on atomic_t. It protects explicitely RCU managed objects which can
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* be visible even after the last reference has been dropped and the object
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* is heading towards destruction.
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*
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* A common usage pattern is:
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*
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* get()
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* rcu_read_lock();
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* p = get_ptr();
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* if (p && !atomic_inc_not_zero(&p->refcnt))
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* p = NULL;
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* rcu_read_unlock();
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* return p;
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*
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* put()
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* if (!atomic_dec_return(&->refcnt)) {
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* remove_ptr(p);
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* kfree_rcu((p, rcu);
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* }
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*
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* atomic_inc_not_zero() is implemented with a try_cmpxchg() loop which has
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* O(N^2) behaviour under contention with N concurrent operations.
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*
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* rcuref uses atomic_add_negative_relaxed() for the fast path, which scales
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* better under contention.
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*
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* Why not refcount?
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* =================
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*
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* In principle it should be possible to make refcount use the rcuref
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* scheme, but the destruction race described below cannot be prevented
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* unless the protected object is RCU managed.
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*
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* Theory of operation
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* ===================
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*
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* rcuref uses an unsigned integer reference counter. As long as the
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* counter value is greater than or equal to RCUREF_ONEREF and not larger
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* than RCUREF_MAXREF the reference is alive:
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*
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* ONEREF MAXREF SATURATED RELEASED DEAD NOREF
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* 0 0x7FFFFFFF 0x8000000 0xA0000000 0xBFFFFFFF 0xC0000000 0xE0000000 0xFFFFFFFF
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* <---valid --------> <-------saturation zone-------> <-----dead zone----->
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*
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* The get() and put() operations do unconditional increments and
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* decrements. The result is checked after the operation. This optimizes
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* for the fast path.
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*
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* If the reference count is saturated or dead, then the increments and
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* decrements are not harmful as the reference count still stays in the
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* respective zones and is always set back to STATURATED resp. DEAD. The
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* zones have room for 2^28 racing operations in each direction, which
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* makes it practically impossible to escape the zones.
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*
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* Once the last reference is dropped the reference count becomes
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* RCUREF_NOREF which forces rcuref_put() into the slowpath operation. The
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* slowpath then tries to set the reference count from RCUREF_NOREF to
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* RCUREF_DEAD via a cmpxchg(). This opens a small window where a
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* concurrent rcuref_get() can acquire the reference count and bring it
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* back to RCUREF_ONEREF or even drop the reference again and mark it DEAD.
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*
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* If the cmpxchg() succeeds then a concurrent rcuref_get() will result in
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* DEAD + 1, which is inside the dead zone. If that happens the reference
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* count is put back to DEAD.
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*
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* The actual race is possible due to the unconditional increment and
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* decrements in rcuref_get() and rcuref_put():
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*
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* T1 T2
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* get() put()
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* if (atomic_add_negative(-1, &ref->refcnt))
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* succeeds-> atomic_cmpxchg(&ref->refcnt, NOREF, DEAD);
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*
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* atomic_add_negative(1, &ref->refcnt); <- Elevates refcount to DEAD + 1
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*
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* As the result of T1's add is negative, the get() goes into the slow path
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* and observes refcnt being in the dead zone which makes the operation fail.
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*
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* Possible critical states:
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*
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* Context Counter References Operation
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* T1 0 1 init()
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* T2 1 2 get()
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* T1 0 1 put()
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* T2 -1 0 put() tries to mark dead
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* T1 0 1 get()
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* T2 0 1 put() mark dead fails
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* T1 -1 0 put() tries to mark dead
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* T1 DEAD 0 put() mark dead succeeds
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* T2 DEAD+1 0 get() fails and puts it back to DEAD
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*
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* Of course there are more complex scenarios, but the above illustrates
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* the working principle. The rest is left to the imagination of the
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* reader.
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*
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* Deconstruction race
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* ===================
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*
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* The release operation must be protected by prohibiting a grace period in
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* order to prevent a possible use after free:
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*
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* T1 T2
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* put() get()
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* // ref->refcnt = ONEREF
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* if (!atomic_add_negative(-1, &ref->refcnt))
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* return false; <- Not taken
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*
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* // ref->refcnt == NOREF
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* --> preemption
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* // Elevates ref->refcnt to ONEREF
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* if (!atomic_add_negative(1, &ref->refcnt))
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* return true; <- taken
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*
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* if (put(&p->ref)) { <-- Succeeds
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* remove_pointer(p);
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* kfree_rcu(p, rcu);
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* }
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*
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* RCU grace period ends, object is freed
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*
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* atomic_cmpxchg(&ref->refcnt, NOREF, DEAD); <- UAF
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*
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* This is prevented by disabling preemption around the put() operation as
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* that's in most kernel configurations cheaper than a rcu_read_lock() /
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* rcu_read_unlock() pair and in many cases even a NOOP. In any case it
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* prevents the grace period which keeps the object alive until all put()
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* operations complete.
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*
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* Saturation protection
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* =====================
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*
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* The reference count has a saturation limit RCUREF_MAXREF (INT_MAX).
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* Once this is exceedded the reference count becomes stale by setting it
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* to RCUREF_SATURATED, which will cause a memory leak, but it prevents
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* wrap arounds which obviously cause worse problems than a memory
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* leak. When saturation is reached a warning is emitted.
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*
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* Race conditions
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* ===============
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*
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* All reference count increment/decrement operations are unconditional and
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* only verified after the fact. This optimizes for the good case and takes
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* the occasional race vs. a dead or already saturated refcount into
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* account. The saturation and dead zones are large enough to accomodate
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* for that.
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*
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* Memory ordering
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* ===============
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*
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* Memory ordering rules are slightly relaxed wrt regular atomic_t functions
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* and provide only what is strictly required for refcounts.
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*
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* The increments are fully relaxed; these will not provide ordering. The
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* rationale is that whatever is used to obtain the object to increase the
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* reference count on will provide the ordering. For locked data
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* structures, its the lock acquire, for RCU/lockless data structures its
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* the dependent load.
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*
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* rcuref_get() provides a control dependency ordering future stores which
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* ensures that the object is not modified when acquiring a reference
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* fails.
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*
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* rcuref_put() provides release order, i.e. all prior loads and stores
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* will be issued before. It also provides a control dependency ordering
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* against the subsequent destruction of the object.
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*
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* If rcuref_put() successfully dropped the last reference and marked the
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* object DEAD it also provides acquire ordering.
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*/
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#include <linux/export.h>
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#include <linux/rcuref.h>
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/**
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* rcuref_get_slowpath - Slowpath of rcuref_get()
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* @ref: Pointer to the reference count
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*
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* Invoked when the reference count is outside of the valid zone.
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*
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* Return:
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* False if the reference count was already marked dead
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*
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* True if the reference count is saturated, which prevents the
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* object from being deconstructed ever.
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*/
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bool rcuref_get_slowpath(rcuref_t *ref)
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{
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unsigned int cnt = atomic_read(&ref->refcnt);
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/*
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* If the reference count was already marked dead, undo the
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* increment so it stays in the middle of the dead zone and return
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* fail.
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*/
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if (cnt >= RCUREF_RELEASED) {
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atomic_set(&ref->refcnt, RCUREF_DEAD);
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return false;
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}
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/*
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* If it was saturated, warn and mark it so. In case the increment
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* was already on a saturated value restore the saturation
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* marker. This keeps it in the middle of the saturation zone and
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* prevents the reference count from overflowing. This leaks the
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* object memory, but prevents the obvious reference count overflow
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* damage.
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*/
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if (WARN_ONCE(cnt > RCUREF_MAXREF, "rcuref saturated - leaking memory"))
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atomic_set(&ref->refcnt, RCUREF_SATURATED);
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return true;
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}
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EXPORT_SYMBOL_GPL(rcuref_get_slowpath);
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/**
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* rcuref_put_slowpath - Slowpath of __rcuref_put()
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* @ref: Pointer to the reference count
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*
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* Invoked when the reference count is outside of the valid zone.
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*
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* Return:
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* True if this was the last reference with no future references
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* possible. This signals the caller that it can safely schedule the
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* object, which is protected by the reference counter, for
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* deconstruction.
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*
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* False if there are still active references or the put() raced
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* with a concurrent get()/put() pair. Caller is not allowed to
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* deconstruct the protected object.
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*/
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bool rcuref_put_slowpath(rcuref_t *ref)
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{
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unsigned int cnt = atomic_read(&ref->refcnt);
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/* Did this drop the last reference? */
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if (likely(cnt == RCUREF_NOREF)) {
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/*
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* Carefully try to set the reference count to RCUREF_DEAD.
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*
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* This can fail if a concurrent get() operation has
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* elevated it again or the corresponding put() even marked
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* it dead already. Both are valid situations and do not
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* require a retry. If this fails the caller is not
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* allowed to deconstruct the object.
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*/
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if (!atomic_try_cmpxchg_release(&ref->refcnt, &cnt, RCUREF_DEAD))
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return false;
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/*
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* The caller can safely schedule the object for
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* deconstruction. Provide acquire ordering.
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*/
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smp_acquire__after_ctrl_dep();
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return true;
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}
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/*
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* If the reference count was already in the dead zone, then this
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* put() operation is imbalanced. Warn, put the reference count back to
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* DEAD and tell the caller to not deconstruct the object.
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*/
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if (WARN_ONCE(cnt >= RCUREF_RELEASED, "rcuref - imbalanced put()")) {
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atomic_set(&ref->refcnt, RCUREF_DEAD);
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return false;
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}
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/*
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* This is a put() operation on a saturated refcount. Restore the
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* mean saturation value and tell the caller to not deconstruct the
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* object.
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*/
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if (cnt > RCUREF_MAXREF)
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atomic_set(&ref->refcnt, RCUREF_SATURATED);
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return false;
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}
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EXPORT_SYMBOL_GPL(rcuref_put_slowpath);
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