[RFC v2 3/3] cxl: Add memory barrier to guarantee TLBI scope
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From: Frederic Barrat <hidden>
Date: 2017-06-22 10:07:06
Subsystem:
the rest · Maintainer:
Linus Torvalds
Revision v2 of 2 in this series.
Revisions (2)
- v2 current
- v3 [diff vs current]
With the hash memory model, all TLBIs become global when the cxl driver is active, i.e. as soon as one context is open. It is theoretically possible to send a TLBI with the wrong scope as there's currently no memory barrier between when the driver is marked as in use, and attaching a context to the device, therefore we are exposed to re-ordering. It is highly unlikely as the use count for the driver is incremented on open() and the attachment to the device happens on a different system call (ioctl) Signed-off-by: Frederic Barrat <redacted> --- include/misc/cxl-base.h | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-)
diff --git a/include/misc/cxl-base.h b/include/misc/cxl-base.h
index b2ebc91fe09a..dcb6d38ab3ad 100644
--- a/include/misc/cxl-base.h
+++ b/include/misc/cxl-base.h@@ -25,12 +25,28 @@ extern atomic_t cxl_use_count; static inline bool cxl_ctx_in_use(void) { - return (atomic_read(&cxl_use_count) != 0); + /* + * This is called when sending an TLBI, to know whether it + * should be global or local. + * + * We need to make sure the PTE update is happening before + * reading the context global flag. Otherwise, reading the + * flag may be re-ordered and happen first, and we could end + * up in a situation where the old PTE is seen by the device, + * but the TLBI is not global. + */ + smp_mb(); + return (atomic_read(&cxl_use_count) != 0); } static inline void cxl_ctx_get(void) { atomic_inc(&cxl_use_count); + /* + * Barrier guarantees that the device will receive all TLBIs + * from that point on + */ + smp_wmb(); } static inline void cxl_ctx_put(void)
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2.11.0