Messages in this thread | | | From | Pranith Kumar <> | Date | Fri, 27 Feb 2015 13:25:25 -0500 | Subject | Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail |
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On Fri, Feb 27, 2015 at 5:06 AM, Will Deacon <will.deacon@arm.com> wrote: > On Fri, Feb 27, 2015 at 05:46:55AM +0000, Pranith Kumar wrote: >> In cmpxchg(), we do a load exclusive on an address and upon a comparison fail, >> we skip the store exclusive instruction. This can result in the exclusive bit >> still set. If there was a store exclusive after this to the same address, that >> will see the exclusive bit set. This should not happen. > > ... and the problem with that is?
Consider the following scenario:
P0 P1 --------------------------------- ldxr x7, [B] // exclusive bit set add x7, x7, #1 str ..., [B] // exclusive bit cleared cmpxchg: ldxr x0, [B] // exclusive bit set cmp x0, #0 // cmp fails b.ne 1f // branch taken stxr x1, [B] // end of cmpxchg 1: stxr x7, [B] // succeeds?
The last store exclusive succeeds since the exclusive bit is set which should not happen. Clearing the exclusive bit before returning from cmpxchg prevents this happening.
Now I am not sure how likely this will happen. One can argue that a cmpxchg() will not happen between an external ldxr/stxr. But isn't clearing the exclusive bit better?
-- Pranith
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