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ARC: Add implicit compiler barrier to raw_local_irq* functions

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ARC irqsave/restore macros were missing the compiler barrier, causing a
stale load in irq-enabled region be used in irq-safe region, despite
being changed, because the register holding the value was still live.

The problem manifested as random crashes in timer code when stress
testing ARCLinux (3.9-rc3) on a !SMP && !PREEMPT_COUNT

Here's the exact sequence which caused this:
 (0). tv1[x] <----> t1 <---> t2
 (1). mod_timer(t1) interrupted after it calls timer_pending()
 (2). mod_timer(t2) completes
 (3). mod_timer(t1) resumes but messes up the list
 (4). __runt_timers( ) uses bogus timer_list entry / crashes in
      timer->function

Essentially mod_timer() was racing against itself and while the spinlock
serialized the tv1[] timer link list, timer_pending() called outside the
spinlock, cached timer link list element in a register.
With low register pressure (and a deep register file), lack of barrier
in raw_local_irqsave() as well as preempt_disable (!PREEMPT_COUNT
version), there was nothing to force gcc to reload across the spinlock,
causing a stale value in reg be used for link list manipulation - ensuing
a corruption.

ARcompact disassembly which shows the culprit generated code:

mod_timer:
    push_s blink
    mov_s r13,r0	# timer, timer
..
    ###### timer_pending( )
    ld_s r3,[r13]       # <------ <variable>.entry.next LOADED
    brne r3, 0, @.L163

.L163:
..
    ###### spin_lock_irq( )
    lr  r5, [status32]  # flags
    bic r4, r5, 6       # temp, flags,
    and.f 0, r5, 6      # flags,
    flag.nz r4

    ###### detach_if_pending( ) begins

    tst_s r3,r3  <--------------
			# timer_pending( ) checks timer->entry.next
                        # r3 is NOT reloaded by gcc, using stale value
    beq.d @.L169
    mov.eq r0,0

    #####  detach_timer( ): __list_del( )

    ld r4,[r13,4]    	# <variable>.entry.prev, D.31439
    st r4,[r3,4]     	# <variable>.prev, D.31439
    st r3,[r4]       	# <variable>.next, D.30246

We initially tried to fix this by adding barrier() to preempt_* macros
for !PREEMPT_COUNT but Linus clarified that it was anything but wrong.
http://www.spinics.net/lists/kernel/msg1512709.html

[vgupta: updated commitlog]

Reported-by/Signed-off-by: Christian Ruppert <christian.ruppert@abilis.com>
Cc: Christian Ruppert <christian.ruppert@abilis.com>
Cc: Pierrick Hascoet <pierrick.hascoet@abilis.com>
Debugged-by/Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Christian Ruppert 2013-04-08 13:05:30 +05:30 committed by Linus Torvalds
parent f465d40d85
commit 79e5f05edc
1 changed files with 8 additions and 4 deletions
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12
arch/arc/include/asm/irqflags.h
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@ -39,7 +39,7 @@ static inline long arch_local_irq_save(void)
" flag.nz %0 \n"
: "=r"(temp), "=r"(flags)
: "n"((STATUS_E1_MASK | STATUS_E2_MASK))
: "cc");
: "memory", "cc");
return flags;
}
@ -53,7 +53,8 @@ static inline void arch_local_irq_restore(unsigned long flags)
__asm__ __volatile__(
" flag %0 \n"
:
: "r"(flags));
: "r"(flags)
: "memory");
}
/*
@ -73,7 +74,8 @@ static inline void arch_local_irq_disable(void)
" and %0, %0, %1 \n"
" flag %0 \n"
: "=&r"(temp)
: "n"(~(STATUS_E1_MASK | STATUS_E2_MASK)));
: "n"(~(STATUS_E1_MASK | STATUS_E2_MASK))
: "memory");
}
/*
@ -85,7 +87,9 @@ static inline long arch_local_save_flags(void)
__asm__ __volatile__(
" lr %0, [status32] \n"
: "=&r"(temp));
: "=&r"(temp)
:
: "memory");
return temp;
}