[PATCH AUTOSEL 5.4 31/34] asm-generic/mmiowb: Allow mmiowb_set_pending() when preemptible()

[Sasha Levin]

From: Will Deacon <will at kernel.org>

[ Upstream commit bd024e82e4cd95c7f1a475a55f99871936c2b2db ]

Although mmiowb() is concerned only with serialising MMIO writes occuring
in contexts where a spinlock is held, the call to mmiowb_set_pending()
from the MMIO write accessors can occur in preemptible contexts, such
as during driver probe() functions where ordering between CPUs is not
usually a concern, assuming that the task migration path provides the
necessary ordering guarantees.

Unfortunately, the default implementation of mmiowb_set_pending() is not
preempt-safe, as it makes use of a a per-cpu variable to track its
internal state. This has been reported to generate the following splat
on riscv:

 | BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
 | caller is regmap_mmio_write32le+0x1c/0x46
 | CPU: 3 PID: 1 Comm: swapper/0 Not tainted 5.8.0-rc3-hfu+ #1
 | Call Trace:
 |  walk_stackframe+0x0/0x7a
 |  dump_stack+0x6e/0x88
 |  regmap_mmio_write32le+0x18/0x46
 |  check_preemption_disabled+0xa4/0xaa
 |  regmap_mmio_write32le+0x18/0x46
 |  regmap_mmio_write+0x26/0x44
 |  regmap_write+0x28/0x48
 |  sifive_gpio_probe+0xc0/0x1da

Although it's possible to fix the driver in this case, other splats have
been seen from other drivers, including the infamous 8250 UART, and so
it's better to address this problem in the mmiowb core itself.

Fix mmiowb_set_pending() by using the raw_cpu_ptr() to get at the mmiowb
state and then only updating the 'mmiowb_pending' field if we are not
preemptible (i.e. we have a non-zero nesting count).

Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Paul Walmsley <paul.walmsley at sifive.com>
Cc: Guo Ren <guoren at kernel.org>
Cc: Michael Ellerman <mpe at ellerman.id.au>
Reported-by: Palmer Dabbelt <palmer at dabbelt.com>
Reported-by: Emil Renner Berthing <kernel at esmil.dk>
Tested-by: Emil Renner Berthing <kernel at esmil.dk>
Reviewed-by: Palmer Dabbelt <palmerdabbelt at google.com>
Acked-by: Palmer Dabbelt <palmerdabbelt at google.com>
Link: https://lore.kernel.org/r/20200716112816.7356-1-will@kernel.org
Signed-off-by: Will Deacon <will at kernel.org>
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
 include/asm-generic/mmiowb.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/mmiowb.h b/include/asm-generic/mmiowb.h
index 9439ff037b2d1..5698fca3bf560 100644
--- a/include/asm-generic/mmiowb.h
+++ b/include/asm-generic/mmiowb.h
@@ -27,7 +27,7 @@
 #include <asm/smp.h>
 
 DECLARE_PER_CPU(struct mmiowb_state, __mmiowb_state);
-#define __mmiowb_state()	this_cpu_ptr(&__mmiowb_state)
+#define __mmiowb_state()	raw_cpu_ptr(&__mmiowb_state)
 #else
 #define __mmiowb_state()	arch_mmiowb_state()
 #endif	/* arch_mmiowb_state */
@@ -35,7 +35,9 @@ DECLARE_PER_CPU(struct mmiowb_state, __mmiowb_state);
 static inline void mmiowb_set_pending(void)
 {
 	struct mmiowb_state *ms = __mmiowb_state();
-	ms->mmiowb_pending = ms->nesting_count;
+
+	if (likely(ms->nesting_count))
+		ms->mmiowb_pending = ms->nesting_count;
 }
 
 static inline void mmiowb_spin_lock(void)
-- 
2.25.1