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[lwip-users] Are sys_arch_protect/unprotect required to nest?

From: Grant Edwards
Subject: [lwip-users] Are sys_arch_protect/unprotect required to nest?
Date: Sun, 14 Nov 2021 17:36:08 -0000 (UTC)
User-agent: slrn/1.0.3 (Linux)

A couple questions about sys_arch_protect/unprotect

 1. Are protect/unprotect calls required to nest?

 2. Does the level passed to unprotect() have to do something?

I think I've found another instance where the original authors of my
sys_arch.c implemented something that meets the requirements stated in
the documentation, but failed to satisfy an unstated requirement.

I also can't figure out how the freeRTOS port satisfies one of the
stated requirements.

According to https://www.nongnu.org/lwip/2_1_x/group__sys__prot.html

    SYS_ARCH_PROTECT Perform a "fast" protect. This could be
    implemented by disabling interrupts for an embedded system or by
    using a semaphore or mutex. The implementation should allow
    calling SYS_ARCH_PROTECT when already protected. The old
    protection level is returned in the variable "lev".


The implemenatation I'm looking at supports both those requirements:

 1. Calling sys_arch_protect() while already protected is allowed.

 2. sys_arch_protect() returns the old protection level.

The protection level is 0 or 1.

sys_arch_protect() disables the scheduler, sets level to 1, returns old level.

sys_arch_unprotect() enables the scheduler, sets level to 0. It
ignores the parameter value, as other ports seem to do.

Looking at the lwIP source code and the freeRTOS port leads me to
believe that there is an unstated requirement that protect/unprotect
calls should nest (should behave like a recursive/rentrant mutex as
described at https://en.wikipedia.org/wiki/Reentrant_mutex).

IOW: 3 successive calls to protect() would require 3 successive calls
to unprotect() before the scheduler is re-enabled.

Simply allowing sys_arch_protect() to be called while already
protected is not sufficient.

Is that correct?

sys_arch_unprotect() parameter value

I can't figure out how the freeRTOS port satisfies the requirement that

    SYS_ARCH_UNPROTECT Perform a "fast" set of the protection level to

AFAICT, the value of "lev" does not affect the behavior at all. The
requirement would seem to be that the value of "lev" can be used to
override the nesting behavior that one would expect from a standard
recursive mutex:

  // context switching allowed here

  sys_prot_t lev;
  lev = sys_arch_prot();
  // context switching disabled here

  // context switching allowed here

I don't see how freeRTOS meets that requirement.

Do I need to implement that requirement?


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