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| author | Andy Lutomirski <luto@kernel.org> | 2020-12-03 21:07:03 -0800 |
|---|---|---|
| committer | Thomas Gleixner <tglx@linutronix.de> | 2020-12-09 09:37:42 +0100 |
| commit | a493d1ca1a03b532871f1da27f8dbda2b28b04c4 (patch) | |
| tree | 61407e37eea602bd636baf5cc44105c85dcb8bf6 | |
| parent | 0477e92881850d44910a7e94fc2c46f96faa131f (diff) | |
| download | linux-a493d1ca1a03b532871f1da27f8dbda2b28b04c4.tar.gz linux-a493d1ca1a03b532871f1da27f8dbda2b28b04c4.tar.bz2 linux-a493d1ca1a03b532871f1da27f8dbda2b28b04c4.zip | |
x86/membarrier: Get rid of a dubious optimization
sync_core_before_usermode() had an incorrect optimization. If the kernel
returns from an interrupt, it can get to usermode without IRET. It just has
to schedule to a different task in the same mm and do SYSRET. Fortunately,
there were no callers of sync_core_before_usermode() that could have had
in_irq() or in_nmi() equal to true, because it's only ever called from the
scheduler.
While at it, clarify a related comment.
Fixes: 70216e18e519 ("membarrier: Provide core serializing command, *_SYNC_CORE")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/5afc7632be1422f91eaf7611aaaa1b5b8580a086.1607058304.git.luto@kernel.org
| -rw-r--r-- | arch/x86/include/asm/sync_core.h | 9 | ||||
| -rw-r--r-- | arch/x86/mm/tlb.c | 10 |
2 files changed, 13 insertions, 6 deletions
diff --git a/arch/x86/include/asm/sync_core.h b/arch/x86/include/asm/sync_core.h index 0fd4a9dfb29c..ab7382f92aff 100644 --- a/arch/x86/include/asm/sync_core.h +++ b/arch/x86/include/asm/sync_core.h @@ -98,12 +98,13 @@ static inline void sync_core_before_usermode(void) /* With PTI, we unconditionally serialize before running user code. */ if (static_cpu_has(X86_FEATURE_PTI)) return; + /* - * Return from interrupt and NMI is done through iret, which is core - * serializing. + * Even if we're in an interrupt, we might reschedule before returning, + * in which case we could switch to a different thread in the same mm + * and return using SYSRET or SYSEXIT. Instead of trying to keep + * track of our need to sync the core, just sync right away. */ - if (in_irq() || in_nmi()) - return; sync_core(); } diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index 11666ba19b62..569ac1d57f55 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -474,8 +474,14 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, /* * The membarrier system call requires a full memory barrier and * core serialization before returning to user-space, after - * storing to rq->curr. Writing to CR3 provides that full - * memory barrier and core serializing instruction. + * storing to rq->curr, when changing mm. This is because + * membarrier() sends IPIs to all CPUs that are in the target mm + * to make them issue memory barriers. However, if another CPU + * switches to/from the target mm concurrently with + * membarrier(), it can cause that CPU not to receive an IPI + * when it really should issue a memory barrier. Writing to CR3 + * provides that full memory barrier and core serializing + * instruction. */ if (real_prev == next) { VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) != |