Re: [RFC PATCH 2/2] ARM: !MMU: v7-M: preemption support

Hi,

On Mon, Dec 30, 2019 at 05:25:27PM +0530, afzal mohammed wrote:

A. Before this change,

A.1
~ # cat /dev/mymisc
[   28.099225] mymisc_open
[   28.101625] mymisc_read: enter

command prompt is not usable (expected), interrupts do happen during
this time.

B. After this change,

B.1
~ # cat /dev/mymisc
[   27.374821] mymisc_open
[   27.377349] mymisc_read: enter

though user will not get control back (as expected as it is fg process),
entering on prompt causes new line, doesn't know what to make out of
this behaviour, this doesn't happen in the A.1 case. Interrupts happen
here as well.

Behaviour B.1 (which is how most of the system behaves) seems due to
serial driver's threaded interrupt handler being able to preempt 'cat'
process, this can't happen in the case of A.1 as preemption doesn't
work.

Also it seems that preemption does happen by tracking __schedule() at
runtime, but issue mentioned in B.2 (below) remains.

So it seems preemption is happening, but is very fragile.

Regards
afzal

B.2
~ # cat /dev/mymisc &
[1] 41 cat /dev/mymisc
~ # [   44.836417] mymisc_open
[   44.838814] mymisc_read: enter

though prompt is available under the control of user, upon typing
anything on the prompt (typed character doesn't get echoed), it crashes
as follows,

[   44.838814] mymisc_read: enter
[   51.710314]
[   51.710314] Unhandled exception: IPSR = 00000006 LR = fffffffd
[   51.717576] CPU: 0 PID: 37 Comm: sh Not tainted 5.5.0-rc4-00004-g2328d01dbd85 #105
[   51.725078] Hardware name: STM32 (Device Tree Support)
[   51.730206] PC is at 0x90195958
[   51.733329] LR is at 0x901c4df3
[   51.736471] pc : [<90195958>]    lr : [<901c4df3>]    psr: 21000000
[   51.742713] sp : 901e5a58  ip : 00000000  fp : 901d89fc
[   51.747911] r10: 00000000  r9 : 00000000  r8 : 00000001
[   51.753143] r7 : 000000a8  r6 : 901e5a58  r5 : 901e5b08  r4 : ffffffff
[   51.759643] r3 : 000000a8  r2 : ffffffff  r1 : 00000001  r0 : 00000001
[   51.766122] xPSR: 21000000
[   51.768866] CPU: 0 PID: 37 Comm: sh Not tainted 5.5.0-rc4-00004-g2328d01dbd85 #105
[   51.776369] Hardware name: STM32 (Device Tree Support)
[   51.781594] [<0800c0c9>] (unwind_backtrace) from [<0800b25b>] (show_stack+0xb/0xc)
[   51.789166] [<0800b25b>] (show_stack) from [<0800b9eb>] (__invalid_entry+0x4b/0x4c)

It is a Usage Fault happening while in thread(user) mode. PC & LR in the
dump is strange in the sense that they do not point to text section.

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