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runtime: avoid write barrier in startpanic_m
startpanic_m could be called correctly in a context where there's a valid G, a valid M, but no P, for example in a signal handler which panics. Currently, startpanic_m has write barriers enabled because write barriers are permitted if a G's M is dying. However, all the current write barrier implementations assume the current G has a P. Therefore, in this change we disable write barriers in startpanic_m, remove the only pointer write which clears g.writebuf, and fix up gwrite to ignore the writebuf if the current G's M is dying, rather than relying on it being nil in the dying case. Fixes #26575. Change-Id: I9b29e6b9edf00d8e99ffc71770c287142ebae086 Reviewed-on: https://go-review.googlesource.com/c/154837 Run-TryBot: Michael Knyszek <mknyszek@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Austin Clements <austin@google.com>
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@ -729,10 +729,13 @@ func fatalpanic(msgs *_panic) {
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// It returns true if panic messages should be printed, or false if
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// the runtime is in bad shape and should just print stacks.
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//
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// It can have write barriers because the write barrier explicitly
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// ignores writes once dying > 0.
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// It must not have write barriers even though the write barrier
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// explicitly ignores writes once dying > 0. Write barriers still
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// assume that g.m.p != nil, and this function may not have P
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// in some contexts (e.g. a panic in a signal handler for a signal
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// sent to an M with no P).
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//
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//go:yeswritebarrierrec
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//go:nowritebarrierrec
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func startpanic_m() bool {
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_g_ := getg()
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if mheap_.cachealloc.size == 0 { // very early
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@ -752,8 +755,8 @@ func startpanic_m() bool {
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switch _g_.m.dying {
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case 0:
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// Setting dying >0 has the side-effect of disabling this G's writebuf.
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_g_.m.dying = 1
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_g_.writebuf = nil
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atomic.Xadd(&panicking, 1)
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lock(&paniclk)
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if debug.schedtrace > 0 || debug.scheddetail > 0 {
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@ -89,7 +89,12 @@ func gwrite(b []byte) {
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}
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recordForPanic(b)
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gp := getg()
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if gp == nil || gp.writebuf == nil {
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// Don't use the writebuf if gp.m is dying. We want anything
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// written through gwrite to appear in the terminal rather
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// than be written to in some buffer, if we're in a panicking state.
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// Note that we can't just clear writebuf in the gp.m.dying case
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// because a panic isn't allowed to have any write barriers.
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if gp == nil || gp.writebuf == nil || gp.m.dying > 0 {
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writeErr(b)
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return
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}
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