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ARC: add compiler barrier to LLSC based cmpxchg
When auditing cmpxchg call sites, Chuck noted that gcc was optimizing away some of the desired LDs. | do { | new = old = *ipi_data_ptr; | new |= 1U << msg; | } while (cmpxchg(ipi_data_ptr, old, new) != old); was generating to below | 8015cef8: ld r2,[r4,0] <-- First LD | 8015cefc: bset r1,r2,r1 | | 8015cf00: llock r3,[r4] <-- atomic op | 8015cf04: brne r3,r2,8015cf10 | 8015cf08: scond r1,[r4] | 8015cf0c: bnz 8015cf00 | | 8015cf10: brne r3,r2,8015cf00 <-- Branch doesn't go to orig LD Although this was fixed by adding a ACCESS_ONCE in this call site, it seems safer (for now at least) to add compiler barrier to LLSC based cmpxchg Reported-by: Chuck Jordan <cjordan@synopsys,com> Cc: <stable@vger.kernel.org> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
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1 changed files with 5 additions and 4 deletions
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@ -25,10 +25,11 @@ __cmpxchg(volatile void *ptr, unsigned long expected, unsigned long new)
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" scond %3, [%1] \n"
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" bnz 1b \n"
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"2: \n"
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: "=&r"(prev)
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: "r"(ptr), "ir"(expected),
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"r"(new) /* can't be "ir". scond can't take limm for "b" */
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: "cc");
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: "=&r"(prev) /* Early clobber, to prevent reg reuse */
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: "r"(ptr), /* Not "m": llock only supports reg direct addr mode */
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"ir"(expected),
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"r"(new) /* can't be "ir". scond can't take LIMM for "b" */
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: "cc", "memory"); /* so that gcc knows memory is being written here */
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return prev;
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}
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