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ldrex/strex breaked by third task ?

thisway_diy

Hi, 

The code is:

1. ldrex r1, [r0]
2. add r1
3. strex r2, r1, [r0]

There are three tasks: A, B, C.

First:  task A run line1, and it gets r1 as 0, now local monitor is "Exclusive Access State",  and then A is pre-empted by B.

B runs line1, line2, line3, it write r0 as 1 ok. The Local monitor is "Open Access State".

C runs line1, the local monitor is "Exclusive Access State" now, and C is pre-empted by A.

A runs line2, line3, and it  success, now r0 is 1.

As above, A and B both success, but r0 is 1 . 

Can I say that : ldrex/strex is breaked by the third task ?

  • +1

    Normally a task switcher should clear the exclusive monitor. That's why B can write the one.
    C is preempted and therefore the exclusive monitor is cleared again.

    But A should see that the write of (r1+1) will not succeed (r2 == 1).

    Anyway your code misses the check of r2 and the branch back to line 1.