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locking/x86/xen: Use try_cmpxchg() in xen_alloc_p2m_entry()
Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old. The x86 CMPXCHG instruction returns success in the ZF flag, so this change saves a compare after CMPXCHG. Also, try_cmpxchg() implicitly assigns old *ptr value to "old" when CMPXCHG fails. There is no need to explicitly assign old *ptr value to the temporary, which can simplify the surrounding source code. No functional change intended. Signed-off-by: Uros Bizjak <ubizjak@gmail.com> Reviewed-by: Juergen Gross <jgross@suse.com> Cc: Juergen Gross <jgross@suse.com> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Link: https://lore.kernel.org/r/20240405083335.507471-1-ubizjak@gmail.com Signed-off-by: Juergen Gross <jgross@suse.com>
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@ -555,7 +555,6 @@ int xen_alloc_p2m_entry(unsigned long pfn)
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/* Separately check the mid mfn level */
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unsigned long missing_mfn;
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unsigned long mid_mfn_mfn;
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unsigned long old_mfn;
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mid_mfn = alloc_p2m_page();
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if (!mid_mfn)
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@ -565,12 +564,12 @@ int xen_alloc_p2m_entry(unsigned long pfn)
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missing_mfn = virt_to_mfn(p2m_mid_missing_mfn);
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mid_mfn_mfn = virt_to_mfn(mid_mfn);
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old_mfn = cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn);
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if (old_mfn != missing_mfn) {
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free_p2m_page(mid_mfn);
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mid_mfn = mfn_to_virt(old_mfn);
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} else {
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/* try_cmpxchg() updates missing_mfn on failure. */
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if (try_cmpxchg(top_mfn_p, &missing_mfn, mid_mfn_mfn)) {
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p2m_top_mfn_p[topidx] = mid_mfn;
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} else {
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free_p2m_page(mid_mfn);
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mid_mfn = mfn_to_virt(missing_mfn);
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}
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}
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} else {
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