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    arm64: spinlock: Ensure forward-progress in spin_unlock_wait · c56bdcac
    Will Deacon 提交于
    Rather than wait until we observe the lock being free (which might never
    happen), we can also return from spin_unlock_wait if we observe that the
    lock is now held by somebody else, which implies that it was unlocked
    but we just missed seeing it in that state.
    
    Furthermore, in such a scenario there is no longer a need to write back
    the value that we loaded, since we know that there has been a lock
    hand-off, which is sufficient to publish any stores prior to the
    unlock_wait because the ARm architecture ensures that a Store-Release
    instruction is multi-copy atomic when observed by a Load-Acquire
    instruction.
    
    The litmus test is something like:
    
    AArch64
    {
    0:X1=x; 0:X3=y;
    1:X1=y;
    2:X1=y; 2:X3=x;
    }
     P0          | P1           | P2           ;
     MOV W0,#1   | MOV W0,#1    | LDAR W0,[X1] ;
     STR W0,[X1] | STLR W0,[X1] | LDR W2,[X3]  ;
     DMB SY      |              |              ;
     LDR W2,[X3] |              |              ;
    exists
    (0:X2=0 /\ 2:X0=1 /\ 2:X2=0)
    
    where P0 is doing spin_unlock_wait, P1 is doing spin_unlock and P2 is
    doing spin_lock.
    Signed-off-by: NWill Deacon <will.deacon@arm.com>
    c56bdcac
spinlock.h 8.2 KB