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[Qemu-devel] [PATCH] spapr_rng: fix race with main loop
From: |
Greg Kurz |
Subject: |
[Qemu-devel] [PATCH] spapr_rng: fix race with main loop |
Date: |
Fri, 11 Mar 2016 19:48:47 +0100 |
User-agent: |
StGit/0.17.1-dirty |
Since commit "60253ed1e6ec rng: add request queue support to rng-random",
the use of a spapr_rng device may hang vCPU threads.
The following path is taken without holding the lock to the main loop mutex:
h_random()
rng_backend_request_entropy()
rng_random_request_entropy()
qemu_set_fd_handler()
The consequence is that entropy_available() may be called before the vCPU
thread could even queue the request: depending on the scheduling, it may
happen that entropy_available() does not call random_recv()->qemu_sem_post().
The vCPU thread will then sleep forever in h_random()->qemu_sem_wait().
This could not happen before 60253ed1e6ec because entropy_available() used
to call random_recv() unconditionally.
This patch ensures the lock is held to avoid the race.
Signed-off-by: Greg Kurz <address@hidden>
---
Thomas,
This is the problem mentioned by Cedric in:
https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg02526.html
Cheers.
--
Greg
hw/ppc/spapr_rng.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/hw/ppc/spapr_rng.c b/hw/ppc/spapr_rng.c
index a39d472b66fd..02d6be49f58e 100644
--- a/hw/ppc/spapr_rng.c
+++ b/hw/ppc/spapr_rng.c
@@ -77,13 +77,13 @@ static target_ulong h_random(PowerPCCPU *cpu,
sPAPRMachineState *spapr,
hrdata.val.v64 = 0;
hrdata.received = 0;
- qemu_mutex_unlock_iothread();
while (hrdata.received < 8) {
rng_backend_request_entropy(rngstate->backend, 8 - hrdata.received,
random_recv, &hrdata);
+ qemu_mutex_unlock_iothread();
qemu_sem_wait(&hrdata.sem);
+ qemu_mutex_lock_iothread();
}
- qemu_mutex_lock_iothread();
qemu_sem_destroy(&hrdata.sem);
args[0] = hrdata.val.v64;
- [Qemu-devel] [PATCH] spapr_rng: fix race with main loop,
Greg Kurz <=