From 79e5f05edcbf85825d19eb8a425cd6c36c6c66f1 Mon Sep 17 00:00:00 2001 From: Christian Ruppert Date: Mon, 8 Apr 2013 13:05:30 +0530 Subject: [PATCH] ARC: Add implicit compiler barrier to raw_local_irq* functions 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] # <------ .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] # .entry.prev, D.31439 st r4,[r3,4] # .prev, D.31439 st r3,[r4] # .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 Cc: Christian Ruppert Cc: Pierrick Hascoet Debugged-by/Signed-off-by: Vineet Gupta Signed-off-by: Linus Torvalds --- arch/arc/include/asm/irqflags.h | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) diff --git a/arch/arc/include/asm/irqflags.h b/arch/arc/include/asm/irqflags.h index ccd84806b62..eac07166820 100644 --- a/arch/arc/include/asm/irqflags.h +++ b/arch/arc/include/asm/irqflags.h @@ -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; } -- 2.41.0