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386afc9114
In UP and non-preempt respectively, the spinlocks and preemption disable/enable points are stubbed out entirely, because there is no regular code that can ever hit the kind of concurrency they are meant to protect against. However, while there is no regular code that can cause scheduling, we _do_ end up having some exceptional (literally!) code that can do so, and that we need to make sure does not ever get moved into the critical region by the compiler. In particular, get_user() and put_user() is generally implemented as inline asm statements (even if the inline asm may then make a call instruction to call out-of-line), and can obviously cause a page fault and IO as a result. If that inline asm has been scheduled into the middle of a preemption-safe (or spinlock-protected) code region, we obviously lose. Now, admittedly this is *very* unlikely to actually ever happen, and we've not seen examples of actual bugs related to this. But partly exactly because it's so hard to trigger and the resulting bug is so subtle, we should be extra careful to get this right. So make sure that even when preemption is disabled, and we don't have to generate any actual *code* to explicitly tell the system that we are in a preemption-disabled region, we need to at least tell the compiler not to move things around the critical region. This patch grew out of the same discussion that caused commits79e5f05edc("ARC: Add implicit compiler barrier to raw_local_irq* functions") and3e2e0d2c22("tile: comment assumption about __insn_mtspr for <asm/irqflags.h>") to come about. Note for stable: use discretion when/if applying this. As mentioned, this bug may never have actually bitten anybody, and gcc may never have done the required code motion for it to possibly ever trigger in practice. Cc: stable@vger.kernel.org Cc: Steven Rostedt <srostedt@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
86 lines
2.5 KiB
C
86 lines
2.5 KiB
C
#ifndef __LINUX_SPINLOCK_UP_H
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#define __LINUX_SPINLOCK_UP_H
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#ifndef __LINUX_SPINLOCK_H
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# error "please don't include this file directly"
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#endif
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#include <asm/processor.h> /* for cpu_relax() */
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/*
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* include/linux/spinlock_up.h - UP-debug version of spinlocks.
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*
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* portions Copyright 2005, Red Hat, Inc., Ingo Molnar
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* Released under the General Public License (GPL).
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*
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* In the debug case, 1 means unlocked, 0 means locked. (the values
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* are inverted, to catch initialization bugs)
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*
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* No atomicity anywhere, we are on UP. However, we still need
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* the compiler barriers, because we do not want the compiler to
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* move potentially faulting instructions (notably user accesses)
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* into the locked sequence, resulting in non-atomic execution.
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*/
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#ifdef CONFIG_DEBUG_SPINLOCK
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#define arch_spin_is_locked(x) ((x)->slock == 0)
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static inline void arch_spin_lock(arch_spinlock_t *lock)
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{
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lock->slock = 0;
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barrier();
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}
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static inline void
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arch_spin_lock_flags(arch_spinlock_t *lock, unsigned long flags)
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{
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local_irq_save(flags);
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lock->slock = 0;
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barrier();
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}
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static inline int arch_spin_trylock(arch_spinlock_t *lock)
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{
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char oldval = lock->slock;
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lock->slock = 0;
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barrier();
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return oldval > 0;
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}
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static inline void arch_spin_unlock(arch_spinlock_t *lock)
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{
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barrier();
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lock->slock = 1;
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}
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/*
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* Read-write spinlocks. No debug version.
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*/
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#define arch_read_lock(lock) do { barrier(); (void)(lock); } while (0)
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#define arch_write_lock(lock) do { barrier(); (void)(lock); } while (0)
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#define arch_read_trylock(lock) ({ barrier(); (void)(lock); 1; })
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#define arch_write_trylock(lock) ({ barrier(); (void)(lock); 1; })
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#define arch_read_unlock(lock) do { barrier(); (void)(lock); } while (0)
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#define arch_write_unlock(lock) do { barrier(); (void)(lock); } while (0)
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#else /* DEBUG_SPINLOCK */
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#define arch_spin_is_locked(lock) ((void)(lock), 0)
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/* for sched.c and kernel_lock.c: */
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# define arch_spin_lock(lock) do { barrier(); (void)(lock); } while (0)
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# define arch_spin_lock_flags(lock, flags) do { barrier(); (void)(lock); } while (0)
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# define arch_spin_unlock(lock) do { barrier(); (void)(lock); } while (0)
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# define arch_spin_trylock(lock) ({ barrier(); (void)(lock); 1; })
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#endif /* DEBUG_SPINLOCK */
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#define arch_spin_is_contended(lock) (((void)(lock), 0))
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#define arch_read_can_lock(lock) (((void)(lock), 1))
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#define arch_write_can_lock(lock) (((void)(lock), 1))
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#define arch_spin_unlock_wait(lock) \
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do { cpu_relax(); } while (arch_spin_is_locked(lock))
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#endif /* __LINUX_SPINLOCK_UP_H */
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