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author | Vineet Gupta <vgupta@synopsys.com> | 2013-09-25 16:53:32 +0530 |
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committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2013-10-18 07:45:45 -0700 |
commit | 0c06a0a693a5baaeacdb4c9485d5d6d490ea8a23 (patch) | |
tree | 6076201217236c94b89d14b7de44ab1992dc72b7 /drivers/watchdog | |
parent | a683a93b1ce0b86944a51a1b8f787aa684836edb (diff) | |
download | linux-3.10-0c06a0a693a5baaeacdb4c9485d5d6d490ea8a23.tar.gz linux-3.10-0c06a0a693a5baaeacdb4c9485d5d6d490ea8a23.tar.bz2 linux-3.10-0c06a0a693a5baaeacdb4c9485d5d6d490ea8a23.zip |
ARC: Workaround spinlock livelock in SMP SystemC simulation
commit 6c00350b573c0bd3635436e43e8696951dd6e1b6 upstream.
Some ARC SMP systems lack native atomic R-M-W (LLOCK/SCOND) insns and
can only use atomic EX insn (reg with mem) to build higher level R-M-W
primitives. This includes a SystemC based SMP simulation model.
So rwlocks need to use a protecting spinlock for atomic cmp-n-exchange
operation to update reader(s)/writer count.
The spinlock operation itself looks as follows:
mov reg, 1 ; 1=locked, 0=unlocked
retry:
EX reg, [lock] ; load existing, store 1, atomically
BREQ reg, 1, rety ; if already locked, retry
In single-threaded simulation, SystemC alternates between the 2 cores
with "N" insn each based scheduling. Additionally for insn with global
side effect, such as EX writing to shared mem, a core switch is
enforced too.
Given that, 2 cores doing a repeated EX on same location, Linux often
got into a livelock e.g. when both cores were fiddling with tasklist
lock (gdbserver / hackbench) for read/write respectively as the
sequence diagram below shows:
core1 core2
-------- --------
1. spin lock [EX r=0, w=1] - LOCKED
2. rwlock(Read) - LOCKED
3. spin unlock [ST 0] - UNLOCKED
spin lock [EX r=0,w=1] - LOCKED
-- resched core 1----
5. spin lock [EX r=1] - ALREADY-LOCKED
-- resched core 2----
6. rwlock(Write) - READER-LOCKED
7. spin unlock [ST 0]
8. rwlock failed, retry again
9. spin lock [EX r=0, w=1]
-- resched core 1----
10 spinlock locked in #9, retry #5
11. spin lock [EX gets 1]
-- resched core 2----
...
...
The fix was to unlock using the EX insn too (step 7), to trigger another
SystemC scheduling pass which would let core1 proceed, eliding the
livelock.
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'drivers/watchdog')
0 files changed, 0 insertions, 0 deletions